This invention concerns the extrusion of plastics materials. The term `plastics materials` as used in this specification is to be construed as including within its scope elastomeric materials such as natural and synthetic rubbers, as well as thermoplastic materials such as polyvinyl chloride, polyethylene, polypropylene and the like.
It is generally accepted that extruders for plastics materials have three main functions to perform namely compounding, which is the mixing of a plastics material, which usually enters the extruder in particulate or granular condition, to produce a homogeneous softened or plastics state extrudable mass; metering, which is the controlling of the output through an extrusion die and which ideally should be for all practical purposes constant; and pressurising which is the generating of sufficient pressure whilst compounding is being carried out to force the softened homogeneous mass through the extrusion die.
Clearly the compounding, metering and pressurizising of the plastics material must be carried out in an extruder which will give a through-put rate which is economically feasible.
Since the properties of plastics materials differ considerably depending upon their chemical and molecular composition there should ideally be an extruder for every different material to be extruded and also there should be a different die head for each of these materials but of course this is not practical economically and thus all extruders are designed in an attempt to reach the best possible compromise so that they can be used to extrude a range of different materials, additionally die head design is also a compromise.
It should also be borne in mind that even if an extruder were always to be used for extruding the same material whose chemical and molecular structure is carefully controlled it is not possible to design the extruder to meet all of the operating conditions ideally. For example, if compounding, metering and pressure creation were achieved to a high degree of accuracy the likelihood is that the residence time of the material in the extruder screw barrel will be uneconomically lengthy or, of course, the extruder size will be undesirably large.
Alternatively whilst it is possible to provide for very high output rates it may, for example, result that the compounding capabilities of the extruder are such that an inferior quality compound will result.
In the case in which the extruder were designed to give very high operating pressures the mechanism would be required to have bearings for the extruder screw which were either very expensive in order to give a long useful life, or are less expensive but suffer from having only a short useful life and thus require a frequent replacement.
Metering per se presents special problems of extruder screw design which if met to give optimum metering capability may well result in the extruder not being acceptably efficient with regard to one or more of the other requirements.
The difficulties briefly outlined above serve to illustrate some of the problems facing the designer of extruders and to illustrate the need for the designer always to decide upon a compromise which will result in the production of a screw extruder which is acceptable. These difficulties are applicable to all types of screw extruders to varying degrees. For example, there are three main types of screw extruder namely the single screw extruder in which a single screw is contained within a single barrel; the multi-screw extruder which comprises two or more intermeshing screws in a single barrel, and the compound extruder which may comprise a number of single or multi-screw extruders which collectively feed a single die or in which the outlet from one extruder feeds the inlet of the next. For simplicity these types of extruder will be referred to in this specification simply as `screw extruders` unless the context requires otherwise for clarity.